Measurements on a CB antenna

Measurements using a miniVNA antenna analyzer

The CB band

I have put up a CB antenna just for fun and to listen to the CB band. The main goal was however to measure this antenna, and if it was possible to use it on radio amateur bands as well.

The antenna is about 6 meters high and is a 5/8 λ tapered aluminium rod, 1.3 m (4.3') collapsed with a nominal impedance of 50 Ω. It's fastened to the outside of the balcony railing which, is a picket fence.

I was very curious to see the electrical parameters (SWR, Z, Rs and Xs), so I connected a miniVNA with my own developed miniVNA program to check these values at the 27 MHz CB region (11 m).

So here's the first result:

(Click on the images to magnify)

SWR is practically 1.1 : 1 at 25.75 MHz, and a nice 1.7 : 1 at 27 MHz. This indicates that the antenna should be a little bit shorter, by 5 % or so.

If the antenne length is exactly 6 meters, it should be shortened to approx. 5.72 meters

This looks very interesting so far.

Amateur bands

So, how does it look like on the amateur bands? Can this antenna be used on the 80, 40, 20, 15 and 10-meter bands?

The 40 meter band

I made another measurement to check this out as well:

It's useless on the 40m band. SWR is way to high with 9.2 : 1 on 7 MHz.

But I mananged to tune it with an ATU, of course, and the antenna might be used even on this band.

The 20 meter band

The SWR on this band looks very promising. Just take at look at 14 MHz, SWR is down to a solid 2:1 ! Why is this so? 14 MHz is approx. the double wavelength of 11 meters, so this makes very good sense. SWR on 28 MHz is also in the vicinity of 2:1.

The 15 meter band

On 15m / 21 the SWR is approx. 2.5:1, and I can manage this band too with an ATU. As mentioned above, by shortening the antenna with 5 %, SWR should also be a better outcome on this band, perhaps lower than 2:1, or even as low as 1.5:1. Pay attention to the SWR at 18.6 MHz, it's almost down to 1:1!

The 2 meter band

The antenna also shows an interesting characteristic on the 2m band:

As with any antenna, this one also has harmonic frequencies at higher frequencies.

At 145 MHz the SWR is very acceptable with 1.7:1. It's not at optimum, but acceptable.

I haven't tried it yet on the 2-meter band though.

Comments

Since this antenna is a purpose-made CB antenna, it is necessary to show the vital electrical data for the CB band. But first a screenshot to show the SWR together with Z, Rs and Xs:

The lower, green line is |Xs|. The blue line is Rs, and the upper brown line is the antenna impedance Z. The actual sign of Xs may be determined by the Phase and / or Ls and Cs from the table below.

At the frequency where SWR is close to 1:1, Xs is very close to 0 as it should be. At this point the antenna is pure resistive and all power is transmitted.

Above this frequency, the value of X is higher, but R is lower and or
Z = R+ jX. The sign of X may be positive or negative depending on capacitance or reactance.

Also notice in the upper right hand corner where the Power Calulation is, only 0.3 W of 4 W is reflected back from the antenna as a result of the low SWR.

The one line table above the diagram shows the actual electrical data at 27 MHz, including Cs and Ls. These values must be cancelled out (by an ATU) in order to make the antenna pure resistive at this frequency.

And then finally, here are all the numbers from the miniVNA program's table when it comes to the CB band:

Conclusion (Next day's adjustment)

The CB antenna experiment was yesterday. Today I decided to adjust the antenna to a "perfect match" on 27 MHz, at the center frequency at 27.175 MHz
(channel 18).

According to my statement above, I would have to adjust it with approx. 5 % (of a half-wave dipole). I figured that my 5/8λ CB antenna really was 6 meters tall, give or take 10 cm (an error margin of roughly ± 1.5 %).

To calculate the estimated adjustment length, I first read the frequency of the lowest SWR, which was 25.75 MHz. For a half wave dipole this would equal to 28 cm (= 11"):

which is 6-5.68= 0.32 [cm], approx. 30 cm (11.8") less height.

Since this is a 5/8 λ and not a 1/2 λ antenna, I have to make another adjustment to my equation:

cm, which I round down to 45 cm (17.7 ') to start with.

After reducing the length with 45-46 cm, this is my new result:

The SWR at the CB center frequency is now at its lowest with an SWR of 1.25, and the adjustment procedure is complete. Bear in mind that yesterday's SWR was a little lower due to a close by building obstacle. This obstacle is now further away, and hence it will influence
the SWR a bit.